Field Ground Detector
The EGDM is an active
system that applies a voltage
to the field circuit.
4-14 Chapter 4 Terminal Board I/O and Equipment Connections
The optional crowbar subsystem limits high negative voltages that can be induced
into the PCM during a pole slip (loss of synchronization) event. Like the de-
excitation module, the crowbar will self-fire based on selective field voltage limits. It
includes a discharge resistor, which may be shared with the de-excitation function
when the two are supplied together. This is typically only provided on generators
with salient poles (hydro applications).
The generator field is an ungrounded system. The field ground detector (EGDM)
detects field leakage resistance between any point in the field circuit of the generator
and ground. The active detection system shown in Figure 4-9 applies a low
frequency square wave voltage with respect to ground, and monitors for current flow
through a high impedance ground resistor. The square wave is ±50 V dc at 0.2 Hz
frequency. If PRV resistors are present, grounds anywhere in the system can be
detected even while the exciter is not being fired, from the PPT secondary to any
point in the generator field.
The ground detector feedback voltage is sent over a fiber-optic link to the DSPX
where it is monitored and alarmed. The DSPX controls the oscillator voltage over an
adjacent fiber-optic link. In the case of redundant control, if the M1 controller fails,
the oscillator in the M2 channel takes over, as commanded by controller C.
One or three EGDMs are mounted next to the power supplies in the power
GEH-6632 EX2100 User's Guide